SIXTY used to be the beginning of the end.Or so it seemed to best-selling writer Judith Viorst, before she hit her 60s. “It’s unbelievable,” she told The Post.”Everybody I know [in their 60s] is fitter now than they’ve ever been in their life. We’re all striving for upper-arm definition and moving our butts on treadmills. And there isn’t a person I know who isn’t doing some kind of volunteer work or activity that goes beyond themselves …

“The 60s,” concludes Viorst, now at the tail end of hers, “have been extremely enjoyable.”

Meet the new 60: It’s yesterday’s 40 – or, if you’re really fit, 30.

Viorst and her contemporaries – like Warren Beatty (63), Sophia Loren (66), Robert Redford (64), Tina Turner (who turns 61 next month) and Gloria Steinem, who recently married for the first time at 66 – are helping to redefine a decade once dreaded as the threshold of decrepitude.

“It used to be that the 60s was seen as the beginning of old age,” says Dr. Rose Dobrof, Brookdale professor of gerontology at New York’s Hunter College.

“Today, most people at 60 think of themselves – and rightly so – as being in the latter part of the middle years.”

The change, Dobrof says, is due largely to two factors: increased longevity and better health.

In the 1800s, she notes, the average life expectancy was in the 40s. These days, when the average 65-year-old woman has about 19 years ahead of her (14 for men), there’s less sense, she says, of “time running out.”

And “study after study,” Dobrof says, shows that the percentage of older people with serious disabilities has been dropping steadily, making this generation’s crop of sixtysomethings the healthiest yet.

“There’s also the fact,” she adds, “that older people are, in general,

better off economically than they were 20 or 30 years ago, and that makes a big difference.

“There are so many good things about being 60,” says Dobrof, a sprightly 75-

year-old who remembers her own sixth decade fondly.

“One writer calls them ‘the elective years.’ Your child-rearing tasks are pretty much complete, for better or worse, and you can choose how you want to spend your time.”

Those who spend at least an hour a day of it exercising are likely to reap the biggest rewards.

At North Carolina’s Appalachian State University, aging and exercise expert David Nieman tracked a group of older women athletes – average age: 73 – and found their hearts, lungs and immune systems functioned as well as those of women half their age.

“These women were putting in an hour and a half a day of exercise,” he says, “and boy, did they look great!”

He says a team in Japan did a similar study of older men and got the same results.

“What every expert in this field will tell you,” Nieman says, “is that for those who exercise, the quality of life is much, much higher.”

Over at the National Institute on Aging in Washington, epidemiologist Jack Guralnik concurs.

“The favorite T-shirt here,” he says, “is, ‘I discovered the fountain of youth,’ and the back of it says, ‘Exercise!'”

He points out that 60 is not the age when the body starts to decline.

“Your body starts going in your 20s and 30s, and there’s a gradual decline from there,” Guralnik says. “The one intervention that seems to have the biggest impact is exercise.”

Then again, concedes Viorst – whose new book of light verse, “Suddenly Sixty and Other Shocks of Later Life,” caps a series that started with her 30s – exercise can take you only so far.

“At my age, people don’t say you look good, they say you look well,” she says. “That’s not a compliment, it’s a diagnosis!”

She recalls how, before going to Los Angeles, she called a friend and asked what she should wear.